The current invention pertains to remote sensing devices, and in particular to fibre optic sensors and communication cables used in such sensing devices, more particularly to methods and apparatus for protecting such sensors, communication cables, and conduits containing such sensors and communication cables from damage resulting from the ambient environment at the remote location.
Sensors for measuring pressure, temperature and temperature profiles, acoustic pressure waves and vibrations, magnetic fields, electric fields and chemical composition potentially can provide valuable information which can be used to characterise oil and gas reservoirs and for managing the cost effective and safe extraction of hydrocarbon reserves from oil and gas wells. Locating such sensors in appropriate positions inside oil and gas wells using conventional methods is difficult and expensive. It is common practice in the oil industry to use wirelines or slicklines to lower sensors into remote downhole positions. While this type of deployment yields valuable information, the procedures make use of expensive equipment and personnel and require that normal production be interrupted. Slickline and wireline procedures also only provide occasional information.
Alternately, it is possible to locate sensors downhole permanently, but the conventional methods for doing this make use of specialist cables which are permanently attached to the production string and complicated mechanical packages such as side-pocket mandrels. This method of installing permanent sensors is extremely expensive and high failure rates are common. When a failure does occur then it is not possible to rectify it without major and extremely costly intervention. In general this is seen as impractical. Repairs can then only be undertaken when a well has to be worked over for other compelling reasons. Even under such conditions rectification of the fault is expensive. It is common experience that conventional pressure sensors such as quartz gauges and silicon strain gauges fail after relatively short periods when at high well bore temperatures and pressures. For example at 135xc2x0 C. or higher the expected lifetimes are short. Reasons for failures are often difficult or impossible to determine, but contributions to failure include failure of the transducer itself, or of downhole electronics, cable degradation and connector contamination.
These well known shortfalls in conventional sensors have led to the development of new types of sensors that can make use of optical fibre technology. The advantages that are invariably expected from this technology include the elimination of downhole electronics.
U.S. Pat. Nos. 5,570,437 and 5,582,064, assigned to Sensor Dynamics, Ltd. of Winchester, England, and which are incorporated herein by reference in their entirety, disclose methods and apparatus for deploying sensors into remote regions of oil wells which can provide permanent monitoring and yet allow cost effective correction in the event that sensors or their associated cables fail. These techniques make use of hydraulic control lines as a xe2x80x9chighwayxe2x80x9d to deliver the sensors to the remote locations. The hydraulic control lines are rugged and provide effective protection for the sensors and their cables against damage during installation. To date the only sensors that have been able to make use of this form of deployment have been fibre optic sensors. They can be extremely small and flexible and can benefit from equally small and flexible cables. This allows such sensors to be moved along hydraulic small bore control lines by fluid drag and to be positioned in remote locations in oil and gas wells. Water is a most convenient fluid for deploying such optical fibre sensors in hydraulic control lines since it is readily available, has excellent low viscosity for pumping and can withstand conditions of very high temperature at high pressure. However, extensive laboratory testing by the assignee of the current invention has established that when optical fibre sensors or optical fibre cables are exposed to water at greater than 70xc2x0 C. and simultaneously to high pressure, for example 2000 psi, then water causes damage to the sensors and also to the cables. It has been shown that water which is in direct contact with the silica fibres can enter into and react with the silica to create highly stressed layers inside the optical fibres. This can also cause failure of the silica through etching. In optical fibre pressure sensors, water ingress has been directly linked to rapid drift in the zero point of optical fiber pressure sensors. At 150xc2x0 C. or greater, the zero point of unprotected fibre optic pressure sensors can change by more than 4000 psi over relatively short time periods. Similarly extreme behaviour has been shown to occur when unprotected optical fibre Bragg gratings are exposed to water under these conditions. Optical fibres have also been shown to change dramatically in length as a result of conditions within the wellbore. Changes greater than 1% have been measured.
In an effort to circumvent these undesirable effects, water has been replaced with a range of other fluids, including silicone or perfluorocarbon fluids and others, some of which are generally regarded as very inert and stable, even at temperatures above 200xc2x0 C. Trials with these fluids showed that damage rates could be reduced but none of the fluids could eliminate damage entirely.
Similar trials with coated fibres showed some improvements, but in no case could a coating or combination of coatings be found which promised long-term survival of optical cables, or which reduced the zero point instability of optical fibre pressure sensors to acceptable levels. Significant improvements were found when optical fibres were coated with carbon, preferably followed by polyimide. However, even the most promising improvements were insufficient to yield a commercially attractive solution. A particular limitation that was identified appears to be associated with pinholes in coatings, which are very difficult to detect and which act as centres for chemical attack that can lead to spreading damage.
This has lead to a widespread search for other coatings that can be applied to the optical fibre sensors and to cables to prevent attack by water or other molecules. Extensive laboratory testing by the assignee of the current invention showed that a wide range of metal coatings failed to protect sensors or cables when exposed to water at high temperatures. Copper, gold and other metals were tried. None survived tests at 250xc2x0 C. in water, over the long term. All coatings were found to affect the temperature sensitivity of the pressure sensor in an undesirable way, increasing the unwanted temperature sensitivity of a pressure sensor by greater than an order of magnitude. In every case additional complications were foreseen in protecting fusion splice joints that inevitably expose bare silica to the environment where optical fibres are spliced.
It has now been established that fibre optic sensors can be effectively protected to provide a stable response at high temperatures and pressures when the sensors are surrounded by silicone oil. This protection can be extended so that sensors can be deployed in remote locations, including downhole locations in oil and gas wells, where the well bore fluids can be highly corrosive.
A recent patent application by SensorDynamics, UK Application Number GB9827735.3, filed Dec. 17, 1998, teaches the use of liquid metals or other liquids in combination with a silica or elastomer capillary. Other materials may also be chosen for the capillary, for example sapphire. The use of metals or other materials that are in the liquid state under the expected operating conditions introduces a series of desirable features. Many Liquid metals readily xe2x80x9cwetxe2x80x9d and hence form a tight interface with silica; some liquid metals, indium, for example are reported to bond to silica. This also enables a highly reflective surface to be produced at a fibre cleaved end-face when xe2x80x9cwettedxe2x80x9d by a liquid metal or where the liquid metal bonds to the silica surface. Liquids cannot support shear stress and therefore do not cause sensors to change their behaviour with changing temperature. For example, polarimetric fibre optic pressure sensors do not become excessively sensitive to changes in temperature. Liquid metals also can readily protect splice regions as well as coated regions of optical fibres and mirrors. Liquid metals can be applied relatively easily to fibres and pumped into capillaries. The use of a liquid interface between the sensor surface and the surrounding capillary further permits the use of multiple coatings on the inside and outside surfaces of the capillary without introducing temperature sensitivity effects in the sensor. In principle the capillary can be used to add protection to cables as well as to sensors.
When pressure sensors are deployed inside hydraulic control lines, referred to as xe2x80x9csensor highwaysxe2x80x9d, it is necessary to ensure that the downhole well bore pressure can be communicated to the interior of the sensor highway where the sensors are located.
The interior of the sensor highway can be filled with a fluid. This fluid can be in the form of a liquid or gas. A useful liquid is an inert oil such as silicone based oil which can be comparatively stable at common bore hole temperatures and pressures. Silicone based fluids can be obtained commercially which are stable at 250xc2x0 C. and higher. The stability of these fluids varies depending on their purity. It can be difficult to guarantee the purity of such fluids over extended periods unless the fluid is enclosed in a hermetically sealed environment. When the highway fluids are allowed to be in direct contact with well bore fluids, then diffusion and convection can occur. This can result in the ingress of water molecules and other species into the highway. In the long term this can result in a hostile environment that attacks even carefully packaged sensors.
It is therefore of great value to devise means for establishing and maintaining the fluid surrounding the sensors and cables in a condition which minimises change in sensors and cables.
The current invention discloses methods and apparatus for creating barriers and segments in a sensor highway utilizing fluids or mechanical devices for any and all of the following purposes:
1. Inhibiting or preventing the ingress of external or reservoir fluids into the highway (The Communication/Barrier Function);
2. Segmenting the highway to form separate sensing regions (The Segmentation Function).
Maximizing the long-term performance of sensors in the highway; and
3. Maximizing the long-term performance of sensors in the highway.
The invention includes apparatus and methods for sensing one or more physical parameters at a remote location while minimizing or eliminating contact between reservoir fluids and the like at the remote location and the sensor used to sense the physical parameters. In one embodiment the apparatus isolates the sensor within a tube containing the sensor. Specifically, apparatus includes a tubing containing a communication cable and a sensor in communication with the cable, the sensor being located within the tubing proximate the remote location. A sealing device is configured to seal a section of the tubing containing the sensor from fluid flow within the tubing, the sealing device being configured to be actuated between a sealing state and a non-sealing state. The apparatus further includes a communication device in fluid communication with the remote location and the section of tubing containing the sensor. A control line is in communication with the sealing device and is configured to actuate the sealing device between the sealing state and the non-sealing state. In a second embodiment, the apparatus is configured to impose a barrier of a fluid between the sensor and the environment at the remote location. Specifically, the latter apparatus includes a first tubing containing a communication cable and a sensor in communication with the cable, the sensor being located within the tubing proximate the remote location. The apparatus further includes a second tubing having a first end in fluid communication with the first tubing proximate the sensor. A fluid barrier reservoir containing a barrier fluid is also provided, the fluid barrier having a first opening in fluid communication with a second end of the second tubing, and a second opening in fluid communication with the remote location.
One method of the present invention includes a method for chemically isolating a sensor from a location at which a parameter is to be measured by the sensor, the location being in a fluid environment. The method includes emplacing within a tube a sensor in signal communication with a communication cable, the sensor being located within a section of the tube proximate the location at which the parameter is to be measured. The section of the tube containing the sensor is isolated from fluid flow within the tube, and the isolated section of the tube containing the sensor is exposed to the fluid environment at the location. The method can further include emplacing within a tube a plurality of sensors in signal communication with the communication cable, the sensors being located within selected sections of the tube proximate associated selected locations at which the parameter is to be measured. The tube selected sections of the tube containing the associated sensors are selectively isolated from fluid flow within the tube, and the isolated selected sections of the tube containing the associated sensors are exposed to the fluid environment at the associated locations.
Another method of the present invention for chemically isolating a sensor from a location at which a parameter is to be measured by the sensor includes emplacing within a tube a sensor in signal communication with a communication cable, the sensor being located within a section of the tube proximate the location at which the parameter is to be measured. A fluid reservoir is placed in fluid communication with the section of the tube containing the sensor, the fluid reservoir further being placed in fluid communication with the fluid environment. The tube is isolated to prevent passage of fluid out of the tube, and a first fluid is passed into the tube to cause the fluid to flow into the fluid reservoir. The method can further include measuring the volume of the first fluid passed down the tube and into the fluid reservoir, and ceasing flowing of the first fluid into the tube when a sufficient volume of the first fluid has been passed down the tube to fill at least a portion of the fluid reservoir.